The present invention relates to improvements in processes for producing hydrogen, carbon monoxide and syngas. More particularly, the present invention provides for improving the yield of hydrogen and carbon monoxide from a hydrocarbon conversion process utilizing a catalytic partial oxidation reaction process.
In many refining and petro-chemical plants, waste streams are generated; typically containing unconverted hydrocarbons, hydrogen, carbon oxides and inerts. These streams are usually present at low pressures and are generally used as fuel in other processes within the complex. Higher profits can be realized if these waste streams are economically converted to higher value products and used as chemical feedstock. Therefore, processes, which can achieve this goal, are of great interest. One such process, with potential for improvement, is the conversion of hydrocarbons to hydrogen and carbon monoxide.
The conversion of hydrocarbons to hydrogen and carbon monoxide containing gases is well known in the art. Examples of such processes include catalytic steam reforming, catalytic autothermal reforming, catalytic partial oxidation and non-catalytic partial oxidation. Each of these processes has advantages and disadvantages and they produce various ratios of hydrogen and carbon monoxide, also known as synthesis gas or syngas.
Most commercial plants for producing hydrogen as the main product are based on the Steam Methane Reforming (SMR) process. An SMR plant consists of a furnace containing several tubes filled with a reforming catalyst. A pre-heated mixture of steam and a source of methane, such as natural gas, is fed to the reactor tubes, wherein they react at temperatures in excess of about 800 C. to form a mixture of H2, CO and CO2 by the highly endothermic reforming reaction. The heat required for the reaction is provided by combustion of fuels with air in the furnace surrounding the tubes. The gas mixture exiting the SMR reactor is passed through another reactor, wherein majority of the CO reacts with steam to form H2 and CO2. In most hydrogen and syngas generation plants, pressure swing adsorption or PSA is used as a process step for the final purification of hydrogen or for splitting the raw gas stream into specified syngas products. The PSA process step generally produces a waste gas stream that carries unrecovered hydrogen, which can be from 10-20% of the hydrogen fed to the PSA process, carbon monoxide and the unconverted hydrocarbon from the original feedstock.
Typically, this waste gas from the PSA only fetches fuel value in the production of hydrogen, carbon monoxide and syngas, as it is recycled back for input into the SMR furnace. In some instances, such as in the methanol production process, the syngas generated by the primary hydrocarbon conversion process is fed to the methanol producing plant, which in turn produces a waste gas stream containing H2, CO, CH4 and CO2. This waste stream is also typically used as fuel in some part of the process.
The present inventors have invented a process, whereby the waste gas stream from the PSA or other processes is further reacted in a partial oxidation reactor and purified such that a significantly higher yield of hydrogen and carbon monoxide is obtained from the overall hydrocarbon conversion process.
Partial oxidation processes convert hydrocarbon containing gases such as natural gas or naphtha to hydrogen, carbon monoxide and other trace components such as carbon dioxide, water, and other hydrocarbons. The process is typically carried out by injecting preheated hydrocarbons and an oxygen-containing gas into a combustion chamber where oxidation of the hydrocarbons occurs with a less than stoichiometric amount of oxygen for complete combustion. This reaction is conducted at very high temperatures such as greater than 1000xc2x0 C. and often in excess of 1300xc2x0 C. and at pressures of up to 150 atmospheres. In some reactions, steam or carbon dioxide can also be injected into the combustion chamber to modify the syngas product and to adjust the ratio of hydrogen to carbon monoxide.
Catalytic partial oxidation is more efficient than non-catalytic partial oxidation in that it uses less oxygen. In this case, the exothermic partial oxidation reaction occurs over a catalyst at temperatures in the range of about 700 to about 1100 C. to produce a reaction product containing high concentrations of hydrogen and carbon monoxide. The catalysts used in these processes are typically noble metals such as platinum or rhodium and other transition metals such as nickel on a suitable support structure.
More recently, partial oxidation processes have been disclosed in which the hydrocarbon gas is contacted with the oxygen-containing gas at high space velocities in the presence of a catalyst such as a metal deposited on a ceramic foam monolith support. The monolith supports are impregnated with a noble metal such as platinum, palladium or rhodium, or other transition metals such as nickel, cobalt, chromium and the like. Typically, these monolith supports are prepared from solid refractory or ceramic materials such as alumina, zirconia, magnesia and the like. During the operation of these reactions, the hydrocarbon feed gases and oxygen-containing gases are contacted with the metal catalyst at temperatures sufficient to initiate the reaction, at a standard gas hourly space velocity (GHSV) of over 10,000 hourxe2x88x921, and often over 100,000 hourxe2x88x921.
The present invention provides for an improved method for recovering hydrogen and carbon monoxide from a waste stream containing these gases along with unconverted hydrocarbons, such as a PSA waste gas stream from a hydrocarbon conversion process. The improvement comprises passing the waste gas stream along with an oxygen-containing gas stream through a monolith catalyst reactor, withdrawing the hydrogen and carbon monoxide from the reactor as a synthesis gas product, or further separating hydrogen and carbon monoxide as separate products. In another embodiment, the gas mixture produced is recycled back into the process or is stored as the final product.
The present invention also provides for means to improve the yield of hydrogen and carbon monoxide from a hydrocarbon conversion process by contacting the waste gas stream from the PSA step of the hydrocarbon conversion process and an oxygen-containing gas stream with a monolith catalyst thereby converting the remaining hydrocarbon to hydrogen and carbon monoxide, and separating the hydrogen and carbon monoxide from the product gas stream.
In a typical process for producing hydrogen, carbon monoxide or syngas, a hydrocarbon such as methane is provided to a hydrocarbon conversion reactor along with steam and optionally an oxygen-containing gas, and is reacted at a temperature of about 800 to 1000xc2x0 C. The resulting products of hydrogen, carbon monoxide, carbon dioxide and unreacted methane are then passed to a shift reactor where further reaction of the unreacted hydrocarbon occurs. This gas stream is then directed to a pressure swing adsorption unit whereby hydrogen is separated as product from the gas mixture. The remaining gas, which does include some hydrogen, carbon dioxide, carbon monoxide, methane and nitrogen is at a low pressure and is directed to the monolith reactor. Oxygen is also fed into the monolith reactor and hydrogen, carbon monoxide and carbon dioxide are removed as products. If necessary, some hydrocarbon fuel may be added to the feed to maintain auto-thermal reactor operation. The hydrogen and carbon monoxide can then be removed from the system for employment in other processes.
Additionally, the present invention also provides for the recovery of the carbon dioxide, which also has utility in other processes. The present invention, therefore, provides for more complete conversion of a hydrocarbon feedstock to the hydrogen and carbon monoxide, and higher recovery of these valuable products compared to that achieved in typical hydrocarbon conversion processes. Process performance is also maintained with the inventive process in terms of the hydrocarbon usage as the reforming and conversion catalysts begin to decay. Thus, full output from a hydrogen or Syngas plant can also be maintained in the face of the reforming and conversion catalyst decay.
Additionally, the present invention further provides for an improved process for the co-production of hydrogen, carbon monoxide and carbon dioxide.